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tissue composed of cells that grow in an abnormal way. Normal tissue is growth-limited, i.e., cell reproduction is equal to cell death. Feedback controls limit cell division after a certain number of cells have developed, allowing for tissue repair but not expansion. Tumor cells are less responsive to these restraints and can proliferate to the point where they disrupt tissue architecture, distort the flow of nutrients, and otherwise do damage.

Tumors may be benign or malignant. Benign tumors remain localized as a discrete mass. They may differ appreciably from normal tissue in structure and excessive growth of cells, but are rarely fatal. However, even benign tumors may grow large enough to interfere with normal function. Some benign uterine tumors, which can weigh as much as 50 lb (22.7 kg), displace adjacent organs, causing digestive and reproductive disorders. Benign tumors are usually treated by complete surgical removal. Cells of malignant tumors, i.e., cancerscancer,
in medicine, common term for neoplasms, or tumors, that are malignant. Like benign tumors, malignant tumors do not respond to body mechanisms that limit cell growth.
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, have characteristics that differ from normal cells in other ways beside cell proliferation. For example, they may be deficient in some specialized functions of the tissues where they originate. Malignant cells are invasive, i.e., they infiltrate surrounding normal tissue; later, malignant cells metastasize, i.e., spread via blood and the lymph system to other sites.

Both benign and malignant tumors are classified according to the type of tissue in which they are found. For example, fibromas are neoplasms of fibrous connective tissue, and melanomas are abnormal growths of pigment (melanin) cells. Malignant tumors originating from epithelial tissue, e.g., in skin, bronchi, and stomach, are termed carcinomas. Malignancies of epithelial glandular tissue such as are found in the breast, prostate, and colon, are known as adenocarcinomas. Malignant growths of connective tissue, e.g., muscle, cartilage, lymph tissue, and bone, are called sarcomas. Lymphomas and leukemiasleukemia
, cancerous disorder of the blood-forming tissues (bone marrow, lymphatics, liver, spleen) characterized by excessive production of immature or mature leukocytes (white blood cells; see blood) and consequently a crowding-out of red blood cells and platelets.
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 are malignancies arising among the white blood cells. A system has been devised to classify malignant tissue according to the degree of malignancy, from grade 1, barely malignant, to grade 4, highly malignant. In practice it is not always possible to determine the degree of malignancy, and it may be difficult even to determine whether particular tumor tissue is benign or malignant.

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An aberrant new growth of abnormal cells or tissues; a tumor.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Clinicopathological features and surgical outcomes of intraductal tubulopapillary neoplasm of the pancreas: a systematic review.
After reclassification, our cohort included 8 unsatisfactory cases, 7 negative for neoplasm cases, 19 lesions of unknown significance, 50 benign neoplasm cases, 6 suspicious for malignancy, and 17 malignant neoplasm cases (Figure).
BPDCN is generally known as a rare and clinically aggressive hematodermic neoplasm occurring in the elderly (median age: 66 years old) with a male-to-female ratio of 3:1.
The cytologic category of oncocytic (Hurthle) cell neoplasm mostly includes low-risk lesions: an institutional experience.
The classification of appendiceal mucinous neoplasm constitutes a very broad diagnostic spectrum ranging from adenoma to mucinous adenocarcinoma (2).
The macrocystic or oligocystic variant of serous cystadenoma is uncommon and difficult to distinguish from mucinous cystic neoplasm by imaging.
Meningiomas are classified according to the neoplasm characteristics, such as tissue invasion, mitotic index, cellular pleomorphism, necrosis and extra neural metastasis, which are malignancy indicators, in addition to the classification in subtypes: meningothelial, fibromatous, transitional, psammomatous, among others (HIGGINS et al., 2017; LOUIS et al., 2016).
Noncontrast MRI identified a neoplasm at the head of the pancreas which was consistent with SPN.
Solid pseudopapillary neoplasm of the pancreas: A single institution experience of 14 cases.
Diagnosis of spindle cell neoplasm of neural origin was established by strong, diffuse positive reactivity against S100 and negative reactivity for both CD68 and smooth muscle actin (ASMA) (Fig 1).
(a-c) H&E staining demonstrates a malignant spindle and epithelioid cell neoplasm with increased mitotic activity ((c), arrow) and necrosis (not shown).
Petrella, "Blastic plasmacytoid dendritic cell neoplasm," in WHO Classification of Tumors of Haematopoietic and Lymphoid Tissues, S.